EXAMPLES OF THE EXPLANATION OF LAWS. 565 



together is generally weaker, the more compound the 

 substance is; and organic products are the most com- 

 pound substances known, those which have the most 

 complex atomic constitution. It is, therefore, upon 

 such substances that the self-propagating power of 

 chemical action is likely to exert itself in the most 

 marked manner. Accordingly, first, it explains the 

 remarkable laws of fermentation, and some of those 

 of putrefaction. f< A little leaven," that is, dough in 

 a certain state of chemical action, impresses a similar 

 chemical action upon " the whole lump." The con- 

 tact of any decaying substance, occasions the decay of 

 matter previously sound. Again, yeast is a substance 

 actually in a process of decomposition from the action 

 of air and water, evolving carbonic acid gas. Sugar 

 is a substance which, from the complexity of its com- 

 position, has no great energy of coherence in its exist- 

 ing form, and is capable of being easily converted (by 

 combination with the elements of water) into carbonic 

 acid and alcohol. Now the mere presence of yeast, 

 the mere proximity of a substance of which the ele- 

 ments are separating from each other, and com- 

 bining with the elements of water, causes sugar to 

 undergo the same change, giving out carbonic acid 

 gas, and becoming alcohol. It is not the elements 

 contained in the yeast which do this. "An aqueous 

 infusion of yeast may be mixed with a solution of 

 sugar, and preserved in vessels from which the air is 

 excluded, without either experiencing the slightest 

 change." Neither does the insoluble residue of the 

 yeast, after being treated with water, possess the 

 power of exciting fermentation. It is not the yeast 

 itself, therefore ; it is the yeast in a state of decom- 

 position. The sugar, which would not decompose and 

 oxidize by the mere presence of oxygen and water, 



